The present invention relates to an apparatus for molding a plastic lens, and more particularly relates to a plastic lens molding apparatus in which a liquid plastic material is injected into a cavity defined between optical surfaces of a pair of molds oppositely arranged, and is then set to produce the plastic lens.
Recently, mold lenses made of plastics are used for most of spectacle lenses in place of glass lenses. In sight curing plastic lenses, there are lenses for short-sightedness, presbyopia and long-sightedness and squint. Among them, optical surfaces of progressive power spectacle lenses and squint curing spectacle lenses are designed to produce prismatic effect in geometric central regions thereof.
To injection mold these kinds of plastic lenses, a pair of molds formed to have predetermined spherical optical surfaces are used. These molds are oppositely placed to define a cavity having a thickness equal to the lens thickness. In this state, a pressure sensitive adhesive tape is applied over the peripheries of the molds. A liquid plastic material is injected into the cavity, and is then solidified to produce a lens. (see Japanese unexamined patent publication 63(1988)-23890).
FIG. 6 illustrates a semi-finished product 1 injection molded. This partially fabricated item undergoes a secondary cutting processing, and thereby a prism is formed at the geometric center of the optical surface of the lens. As shown in FIG. 7, a jig 2 is attached to the convex spherical surface 1a of the semi-finished item 1. The attached surface 4 of the jig 2 is inclined an angle .theta. to a reference surface 3. Accordingly, the semi-finished item 1 is attached with the center line 5 inclined at the angle .theta. to the reference surface 3. The semi-finished item 1 is cut by setting a center of curvature on the axis of the jig 2 to form a spherical optical surface 6 with an arc R. Polishing is applied to the optical surface 6 using aluminum oxide, and then the jig 2 is removed. Thus, a plastic lens 7 is, as shown in FIG. 9, formed, having a prism formed at a geometric central region of the optical surface.
This earlier attempt needs many fabrication steps: the jig is attached to the semi-finished item after the latter has been injection molded; then, the spherical optical surface is formed; and finally the item is subjected to polishing. About a half of the semi-finished item is removed to produce a chip, and this deteriorates yield of the lens, resulting in a considerable rise in production cost.